From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E6A28299A85 for ; Fri, 17 Oct 2025 18:06:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760724381; cv=none; b=VvLUfIyLClsLPBuWsVOpCaw284s9G408OI9GOSxpuF4v8hvS6FB7XEDUgsbUrFkmMpmt8eXSwiKedljSrodqPEw3OpczamMn5J4mAUdvl/taUgrKsm/Oy3eEo/CfLhMeM7oQ4ijaELRsemNNIpcU2S1v1Qfc+rYCoZQWon1hERE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760724381; c=relaxed/simple; bh=PRpIF8coTvyiXK1iielJL7vqdi95WfSextDIfNHf+BM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=IazHa0/i3Qkznr8SwkvmjADhjtL1N/HTKoDq/Rc8ocBpljvs5S2PG6waVB5Cx6Nq2EQpDkaPJOrYE6ROjf/BmFr+KKpAwPDjDdGEkeQriFAqw7GPqsTFmVhYUeZfYgIeQibW/GqPiMskAgHfRef1bxttRqGCYIGPmSXGRGzrYtM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 76468C4CEE7; Fri, 17 Oct 2025 18:06:19 +0000 (UTC) Date: Fri, 17 Oct 2025 19:06:16 +0100 From: Catalin Marinas To: Huang Ying Cc: Will Deacon , Anshuman Khandual , Ryan Roberts , Gavin Shan , Ard Biesheuvel , "Matthew Wilcox (Oracle)" , Yicong Yang , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] arm64, mm: avoid always making PTE dirty in pte_mkwrite() Message-ID: References: <20251015023712.46598-1-ying.huang@linux.alibaba.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251015023712.46598-1-ying.huang@linux.alibaba.com> On Wed, Oct 15, 2025 at 10:37:12AM +0800, Huang Ying wrote: > Current pte_mkwrite_novma() makes PTE dirty unconditionally. This may > mark some pages that are never written dirty wrongly. For example, > do_swap_page() may map the exclusive pages with writable and clean PTEs > if the VMA is writable and the page fault is for read access. > However, current pte_mkwrite_novma() implementation always dirties the > PTE. This may cause unnecessary disk writing if the pages are > never written before being reclaimed. > > So, change pte_mkwrite_novma() to clear the PTE_RDONLY bit only if the > PTE_DIRTY bit is set to make it possible to make the PTE writable and > clean. > > The current behavior was introduced in commit 73e86cb03cf2 ("arm64: > Move PTE_RDONLY bit handling out of set_pte_at()"). Before that, > pte_mkwrite() only sets the PTE_WRITE bit, while set_pte_at() only > clears the PTE_RDONLY bit if both the PTE_WRITE and the PTE_DIRTY bits > are set. > > To test the performance impact of the patch, on an arm64 server > machine, run 16 redis-server processes on socket 1 and 16 > memtier_benchmark processes on socket 0 with mostly get > transactions (that is, redis-server will mostly read memory only). > The memory footprint of redis-server is larger than the available > memory, so swap out/in will be triggered. Test results show that the > patch can avoid most swapping out because the pages are mostly clean. > And the benchmark throughput improves ~23.9% in the test. > > Fixes: 73e86cb03cf2 ("arm64: Move PTE_RDONLY bit handling out of set_pte_at()") > Signed-off-by: Huang Ying > Cc: Catalin Marinas > Cc: Will Deacon > Cc: Anshuman Khandual > Cc: Ryan Roberts > Cc: Gavin Shan > Cc: Ard Biesheuvel > Cc: "Matthew Wilcox (Oracle)" > Cc: Yicong Yang > Cc: linux-arm-kernel@lists.infradead.org > Cc: linux-kernel@vger.kernel.org > --- > arch/arm64/include/asm/pgtable.h | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index aa89c2e67ebc..0944e296dd4a 100644 > --- a/arch/arm64/include/asm/pgtable.h > +++ b/arch/arm64/include/asm/pgtable.h > @@ -293,7 +293,8 @@ static inline pmd_t set_pmd_bit(pmd_t pmd, pgprot_t prot) > static inline pte_t pte_mkwrite_novma(pte_t pte) > { > pte = set_pte_bit(pte, __pgprot(PTE_WRITE)); > - pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY)); > + if (pte_sw_dirty(pte)) > + pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY)); > return pte; > } This seems to be the right thing. I recall years ago I grep'ed (obviously not hard enough) and most pte_mkwrite() places had a pte_mkdirty(). But I missed do_swap_page() and possibly others. For this patch: Reviewed-by: Catalin Marinas I wonder whether we should also add (as a separate patch): diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c index 830107b6dd08..df1c552ef11c 100644 --- a/mm/debug_vm_pgtable.c +++ b/mm/debug_vm_pgtable.c @@ -101,6 +101,7 @@ static void __init pte_basic_tests(struct pgtable_debug_args *args, int idx) WARN_ON(pte_dirty(pte_mkclean(pte_mkdirty(pte)))); WARN_ON(pte_write(pte_wrprotect(pte_mkwrite(pte, args->vma)))); WARN_ON(pte_dirty(pte_wrprotect(pte_mkclean(pte)))); + WARN_ON(pte_dirty(pte_mkwrite_novma(pte_mkclean(pte)))); WARN_ON(!pte_dirty(pte_wrprotect(pte_mkdirty(pte)))); } For completeness, also (and maybe other combinations): WARN_ON(!pte_write(pte_mkdirty(pte_mkwrite_novma(pte)))); I cc'ed linux-mm in case we missed anything. If nothing raised, I'll queue it next week. Thanks. -- Catalin